1. Field of the Invention
The present invention relates to a semiconductor dynamic quantity sensor for detecting a dynamic force.
2. Description of Related Art
A semiconductor dynamic quantity sensor includes a semiconductor substrate and a bridge circuit. The semiconductor substrate has a diaphragm that changes its shape in accordance with an application of a dynamic force, such as a pressure and acceleration force. The bridge circuit has gauge resistors formed on the substrate. Resistances of the gauge resistors vary as the shape of the diaphragm 14 changes. The sensor produces a sensor output and a diagnostic output through the use of the bridge circuit for a fault diagnosis that is performed based on a comparison of the outputs.
In such a sensor, the outputs for the dynamic quantity measurement and for the fault diagnosis can be produced by a single bridge circuit. As a result, the sensor is reduced in size.
The applicant filed a related Japanese patent application No. JP-2001-221697. FIG. 9 shows an equivalent circuit of a bridge circuit described in the related art. The bridge circuit has a Wheatstone bridge circuit constructed of four gauge resistors Ra, Rb, Rc, and Rd that are connected in series to form a quadrangular closed circuit. Resistors of wiring patterns H1 to H10 between the gauge resistors are shown.
Terminals A, B, C, and D are electrically connected to respective junctions of the wiring patterns H1 and H10, H31 and H31, H5 and H6, and H81 and H82. The terminals A and D are used for inputting an input voltage (input signal) Vin. The terminals B and C are used for outputting a first differential voltage (output signal) Vout1. The input terminals A and D are electrically connected to a power supply (first electric potential) and the ground (second electric potential), respectively.
Each of the four gauge resistors Ra to Rd is equally divided into two divisional gauge resistors. For instance, the gauge resistor Ra, which is provided between the input terminal A and the output terminal B, is divided into the divisional gauge resistors Ra1 and Ra2. Other gauge resistors Rb to Rd are divided into two gauge resistors Rb1 and Rb2, Rc1 and Rc2, and Rd1 and Rd2, respectively in the same manner as the gauge resistor Ra.
The gauge resistor Ra has a first midpoint between the divisional gauge resistors Ra1 and Ra2. The gauge resistor Rb has a second midpoint between the divisional gauge resistors Rb1 and Rb2, and the second midpoint is electrically connected to a middle terminal B1. The gauge resistor Rc has a third midpoint between the divisional gauge resistors Rc1 and Rc2. The gauge resistor Rd has a fourth midpoint between the divisional gauge resistors Rd1 and Rd2, and the fourth midpoint is electrically connected to a middle terminal C1.
The middle terminals B1 and C1 make a combination of the midpoints at which an equal electric potential is measured when no pressure is applied to the bridge circuit. The combination of the middle terminals B1 and C1 is used as the output terminals for the diagnosis. A second potential difference Vout2 between the middle terminals B1 and C1 is used as the diagnostic output to detect a fault of the sensor.
When the diaphragm changes its shape by applying the dynamic force in condition that the input signal Vin is inputted into the bridge circuit, the four gauge resistors Ra to Rd lose a balance in electrically. The first differential voltage (output signal) Vout1 varies in response to the amount of the applied dynamic force. The first differential voltage Vout1 is supplied to an external circuit to detect intensity of the dynamic quantity.
The fault is detected by that the first differential voltage Vout1 is compared with the second differential voltage Vout2. The first differential voltage Vout1 is used as the sensor output. Since each divisional gauge resistors Ra1 to Rd2 has the same resistance, a first ratio between the divisional gauge resistors Rd1 and Rd2 is 1:1, and a second ratio between the divisional gauge resistors Rb1 and Rb2 is 1:1. Accordingly, in a normal condition, the second differential voltage Vout2 is always a half voltage of the first differential voltage Vout1.
However, in an unusual condition that an irregular stress is applied to the diaphragm or the divisional gauge resistors Ra1 to Rd2 are broken, the first ratio and the second ratio are deviated from 1:1. As a result, in the unusual condition, the second differential voltage Vout2 for the diagnosis is not a half voltage of the first differential voltage Vout1. That is, a relation between the differential voltages Vout1 and Vout2 in the unusual condition is different from the relation in the normal condition. This provides the judgment of the fault in the sensor.
Offset voltages exist in both the first differential voltage Vout1 and the second differential voltage Vout2 in the bridge circuit. The offset voltages, which are generated by the first and second differential voltages Vout1 and Vout2 when the dynamic force is not applied to the diaphragm, cause an error of detection. If resistors of the wiring patterns H1 to H10 that connect between the divisional gauge resistors Ra1 to Rd2 are zero or the identical in the bridge circuit, resistances of the bridge circuit are determined by a pattern of the circuit or a circuit layout. Further, if each of the resistances of the divisional gauge resistors is identical, the offset voltage does not exist in the sensor output and the diagnostic output.
However, in reality, resistances of the wiring patterns H1 to H10 exist in each of the divisional gauge resistors Ra1 to Rd2 and between the terminals A, B, C, D, B1, and B2. If the resistances of the wiring patterns H1 to H10 are identical with each other, the offset voltage is equal to zero. However, it is difficult to adjust the resistances of the wiring patterns to be equal to each other because of the pattern of the wiring patterns and the circuit layout.
In such a sensor, an external resistor is connected to the bridge circuit, and a resistance of the external resistor is adjusted by a trimming operation so that the offset voltage becomes a certain voltage. However, it is difficult to adjust the two offset voltages with respect to the sensor output and the diagnostic output by the connection of the external resistor.
FIG. 10 shows an equivalent circuit of the bridge circuit shown in FIG. 9 along with the external resistors. Suppose that the offset voltage of the second differential voltage Vout2 is equal to zero, and the offset voltage of the first differential voltage Vout1 is a certain voltage that is not equal to zero.
External resistors Rg1 and Rg2 are provided to the bridge circuit to adjust the offset voltage of the first differential voltage Vout1. This causes a variation of the electric potential of the output terminal B with reference to the output terminal C-because an electrical current between the terminals A and D varies. As a result, the offset voltage of the first differential voltage Vout1 is adjusted to be equal to zero.
However, an electrical potential of the terminal B1 also varies with reference to the electrical potential of the terminal C1 due to the variation of the electrical current between the terminals A and D by the connection of the external resistors Rg1 and Rg2. As a result, the offset voltage of the second differential voltage Vout2 for the diagnostic output turns not equal to zero.
Similarly, the two offset voltages Vout1 and Vout2 are not adjusted simultaneously even if external resistors Rv1 and Rv2 are provided to a high voltage potential side of the bridge circuit. That is, either the offset voltage Vout1 or Vout2 in the bridge circuit is adjusted with the external resistors, but the two offset voltages Vout1 and Vout2 are not adjusted to be equal to zero simultaneously.